Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore, other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersions used in coating processes are usually in a concentrated form and contain a significant quantity of nonionic surfactant, e.g. 6-8 weight percent, as taught in Marks et al., U.S. Pat. No. 3,037,953. Similar dispersions and coating processes are also used for making coatings of melt-processible fluoropolymers.
For some specialized fluoropolymer coating dispersions, common nonionic surfactants such as alkyl phenol ethoxylates or aliphatic alcohols ethoxylates for stabilization are unsuitable. One such dispersion is a formulation in which chromic acid is used as a component of a primer coating to increase adherence to metal substrates. Chromic acid has been shown to attack nonionic surfactants such as TRITON® X-100, an alkyl phenol ethoxylate. Historically, an anionic surfactant such as sodium lauryl sulfate has been used in this primer formulation because it is stable in the presence of chromic acid.
Fluorosurfactants are typically used as an ingredient in the dispersion polymerization of fluoropolymers, the fluorosurfactants functioning as a non-telogenic dispersing agent. For example, an early description of this use of fluorosurfactants is found in U.S. Pat. No. 2,559,752 to Berry. However because of environmental concerns and because fluorosurfactants are expensive, processes have been developed for reducing and recovering fluorosurfactant from aqueous fluoropolymer dispersions.
One common method is to remove fluorosurfactant by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls) and U.S. Pat. No. 6,833,403 (Bladel et al.) For effective removal, such dispersions are stabilized with a nonionic surfactant, such as alkyl phenol ethoxylates or aliphatic alcohol ethoxylates as disclosed in U.S. Pat. No. 3,037,953 to Marks et al.; U.S. Pat. No. 6,153,688 to Miura et al.; and U.S. Pat. No. 6,956,078 to Cavanaugh et al. Dispersions stabilized with nonionic surfactant are used since removal of the fluorosurfactant without nonionic surfactant being present generally results in coagulation of the dispersion.
If it is attempted to use an anion exchange process for the fluoropolymer dispersions discussed above that are stabilized with an anionic hydrocarbon surfactant such as sodium lauryl sulfate instead of nonionic surfactant, the anionic hydrocarbon surfactant will be removed from the dispersion together with the fluorosurfactant causing coagulation of the dispersion.
Improved stabilized concentrated fluoropolymer dispersions with reduced fluorosurfactant content are desired.